We report two unrelated cases of Costello syndrome, presenting with poor postnatal growth, mild mental retardation, poor feeding, curly hair, coarse characteristic face, loose skin, hypotonia, and cardiac involvement. Nasal papilloma and acanthosis nigricans were the most characteristic features of this syndrome. Both cases had atrial fibrilation from infancy to early childhood. One patient had hypertonia in the lower extremities and pes equinovarus, while the other had hypotonia and pes planovalgus.